The overall objective of this proposal is to develop a protocol for designing bioactive, biomimetic polymer-inorganic composites for replacement of hard tissue in the body such as enamel, dentin and bone. The approach is to study natural composites in order to determine the key properties for structural superiority, to understand the structure-property relationships in biological systems, and to mimic the properties or structures as much as is feasible in developing efficient bioactive designs for structural repair in the body, especially in dental applications. It is proposed to determine the apparent toughness, strength and crack propagation mechanisms in synthetic and natural inorganic-polymer composites in order to develop a methodology for biomedical composite design. This study proposes to select several natural polymer-inorganic composites such as the stone crab chelae and mollusk shell in order to determine the mechanical properties such as fracture toughness, hardness, strength and interface integrity as a function of the micro-structure and macro-structure. In fact, a main focus of this study is a determination of the structure, integrity, and mechanical behavior of the interface between phases. Unique capabilities are available at the University of Florida for the control of the geometry and chemistry of inorganic-organic interfaces. Thus, a concerted effort will be devoted to the identification of the role of the interfaces in various biological structures in order to replicate the behavior in designed composites. Some unique features of the present study are the application of fractographic analysis of failure origins, fractal analysis to characterize the micro-structure on the failure surfaces, and the identification of the chemistry and mechanics of the interfaces. The synthetic composites to be studied include existing bioactive ceramic-polymer composites, collagen-polymer-inorganic composites and proposed composites based on the studies of biological and existing composites. The protocol developed will be useful in the design of hard tissue for the body to aid in the development of replacement parts that are both bioactive and matched in mechanical properties for the replacement tissue. Although these designed composites have a particular emphasis on dental applications, the principles developed should be applicable to a much broader venue.